Electrospark deposition process for oxidation resistant coating of cooling hole

ABSTRACT

A method of providing an oxidation resistant coating is disclosed. The method includes providing a substrate having a first surface and cooling holes. A portable coating device includes electro-spark deposition (ESD) equipment and an ESD torch connected with the ESD equipment. The ESD torch has an inert gas source and a rotary electrode conductive material. The rotary electrode is positioned within the ESD torch, and is shielded by an inert gas. The rotary electrode applies a compositionally controlled protective coating to the first surface of the substrate. Then the rotary electrode is inserted into the cooling hole and generates an electrospark between rotary ESD electrode and the substrate to form a rounded edge and deposit a coating of electrode material alloy at a cooling hole edge.

FIELD OF THE INVENTION

This application generally relates to gas turbine components. Theapplication relates more specifically to the use of an electrosparkdeposition process to apply an oxidation resistant coating to a coolinghole of a gas turbine component.

BACKGROUND OF THE INVENTION

Many component parts of a gas turbine engine include cooling holes foractive cooling of engine sections located downstream of the turbinesection. The rising combustor exit temperatures in gas turbine enginesnecessitate active cooling to avoid thermal failure. For example, atransition piece for a gas turbine engine typically includes an integralframe portion surrounding an opening at a downstream end where thetransition piece connects to the turbine stage. An exemplary transitionpiece is described in U.S. Pat. No. 5,414,999.

Under the high temperature operating conditions of the gas turbineengine, fracture or cracks may occur around the cooling holes located onportions of the frame. Failure analysis has revealed that such cracksform perpendicular to the inner surface of the frame, which indicatesthat thermal stresses played a role in forming the cracks occurring atthe cooling hole. Grain boundary oxidation and thermal fatigue arepotential causes of such cracking. Cracks initiated at the cooling holeson the aft-facing end of the frame and propagated into the body of theframe. Analysis indicated that cracks in the cooling holes of the framefollowed the oxidized grain boundaries. Currently there is no process toprevent crack initiation at the aft end of the cooling holes.

There is a need for local reinforcement of cooling holes in a gasturbine component. There is also a need to provide enhanced oxidationresistance around the cooling holes to reduce oxidation and crackingalong grain boundaries.

Intended advantages of the disclosed systems and/or methods satisfy oneor more of these needs or provide other advantageous features. Otherfeatures and advantages will be made apparent from the presentspecification. The teachings disclosed extend to those embodiments thatfall within the scope of the claims, regardless of whether theyaccomplish one or more of the aforementioned needs.

BRIEF DESCRIPTION OF THE INVENTION

One embodiment the disclosure relates to a method of providing anoxidation resistant coating. The method includes providing a substratehaving a first surface and at least one cooling hole; providing aportable coating device including: electro-spark deposition (ESD)equipment, and an ESD torch electrically connected with the ESDequipment; the ESD torch including: an inert gas source; and a rotaryelectrode including a conductive material, the rotary electrode disposedwithin the ESD torch, the rotary electrode shielded by an inert gas; andthe rotary electrode applies a compositionally controlled protectivecoating to the first surface of the substrate; then, inserting therotary electrode at least partially into the cooling hole; generating anelectrospark between rotary ESD electrode and the substrate to form arounded edge and deposit a coating of electrode material alloy at acooling hole edge.

Another embodiment relates to a system for depositing an oxidationresistant coating on a cooling hole edge in a substrate. The systemincludes an electrospark device and an electrode removably supported inthe electrode holder. The electrospark device is configured to apply acoating of a material when inserted into a cooling hole in the substrateand placed into contact with the metal substrate. A rotary electrode isdisposed within the ESD torch. The rotary electrode is shielded by aninert gas. The rotary electrode applies a compositionally controlledprotective coating to the substrate at an edge of the cooling hole inresponse to an electrospark generated by an electrical current throughthe rotary electrode.

The present disclosure includes a method to enhance the oxidationresistance of the cooling hole exit locally by applying an ESD processwith the electrode having an appropriate tip profile. The ESD processestablishes an electrospark between the rotary electrode and the holeexit. Heat from the electrospark deposition softens and deforms theupper corner of the cooling hole to form a rounded edge with an alloycoating that provides improved resistance to oxidation of the substratemetal.

An advantage of the disclosed method is a reduction in the number ofturbine components discarded or scrapped.

Another aspect is the ability to heat and deform the top corners of acooling hole to a rounded shape using ESD.

Still another aspect of the disclosure is the ability to build up an ESDcoating layer having a superior resistance to oxidation of the metalsubstrate.

Alternative exemplary embodiments relate to other features andcombinations of features as may be generally recited in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-sectional view of a prior art cooling hole.

FIG. 2 shows a cross-sectional view of a rounded, oxidation resistantcooling hole formed with the ESD process.

FIG. 2A is an enlarged section view of the rounded corner of FIG. 2.

FIG. 3 shows a schematic arrangement for performing the ESD process on acooling hole.

FIG. 4 shows a transition piece aft frame portion of a gas turbineengine.

FIG. 4A shows an enlarged view of section 4A in FIG. 4 showing crackingaround cooling holes in the transition frame portion.

FIG. 5 shows a flow chart of the method of enhancing oxidationresistance of cooling holes on combustion components of a gas turbineengine.

FIG. 6 shows a rounded, coated cooling hole exit withoxidation-resistant coating.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1 a cross-sectional view of a prior art cooling holeis shown. A cooling hole 10 passes through a metal frame substrate 12.An edge 14 of the cooling hole 10 appears at each of the top surface 16or the bottom surface 18 of metal frame substrate 12. Cooling hole 10 isformed in metal frame substrate 12 to provide air flow therethrough forcooling metal substrate 12 in harsh, high temperature environments,e.g., in a gas turbine engine transition piece (not shown). Edges 14 aresubject to oxidation when exposed to harsh, high temperatureenvironments such as are present in a gas turbine engine. The oxidizedsubstrate material adjacent to cooling holes 10 results in cracksforming in metal substrate 12 around cooling holes 10. In particular, atthe aft-facing side end of metal substrate 12 cracks are prone to form.

Referring next to FIGS. 2 and 2A, a cooling hole 10 is shown which hasbeen treated by the ESD coating process described in greater detailbelow. Metal frame substrate 12 has a rounded edge 20 with an oxidationresistant coating 22 adjacent top surface 16 from applying the ESDcoating process. Edge 14 on bottom surface 18 has not been exposed tothe ESD coating process, and as a result edge 14 remains a sharp cornerconfiguration without a rounded, coated edge. In one embodiment,oxidation resistant coating 22 may be about 2 mils thick oversubstantially the entire surface 24 of rounded edge 20. The coatingthickness may vary more or less depending on the particular geometry ofthe cooling holes, e.g., in some embodiments the coating thickness maybe as thick as 3 mils.

Referring next to FIG. 3, an exemplary arrangement for rounding andcoating edge 20 in cooling hole 10 is shown. An ESD torch 21 iselectrically connected to the ESD equipment by an electrical connection(not shown). Electrical current allows the ESD torch 21 to generate aspark to melt a portion of a rotary electrode 30. The ESD torch 21 isused to apply a compositionally controlled protective coating 22 to thesubstrate 12 at an edge of cooling hole 10. In one embodiment, the ESDtorch 21 and associated equipment includes a conventional ESD powersource, which incorporates either a series of capacitors or a siliconcontrolled rectifier coupled with isolated gate bipolar transistorswitches. The deposition rate for the ESD torch 21 varies depending onthe application speed determined by the user.

The rotary electrode 30 having a partially tapered tip portion 32 isinserted at least partially into cooling hole 10 through metal framesubstrate 12 adjacent top side 16. In one embodiment tip portion 32includes a transition portion 33 transitioning from the diameter ofrotary electrode 30. The diameter of rotary electrode 30 is slightlylarger than the diameter of cooling hole 10, while a smaller diametertip portion 32 is less than the diameter of cooling hole 10, to permitpartial insertion of tip portion 32 into cooling hole 10. In otherembodiments the shape of tip portion 32 may have a geometry tailored forforming a predetermined geometry of the cooling hole exit, for example,a rounded edge. The substrate 12 may be, e.g., a piece of combustionhardware, e.g., a transition piece aft picture frame 15 (also see FIG.4). The edge of the transition piece aft picture frame 15 of the turbineengine includes a plurality of cooling holes 20 (see FIG. 4).

A first shielding gas flow 34 is directed at tip portion 32 to providean inert gas curtain around the deposition site at edge 20. A secondshielding gas flow 36 may also be directed at tip portion 32 throughcooling hole 10 from bottom surface 18. Shielding gas is well known tothose skilled in weld process, such as electrospark deposition, andprevents oxygen and other gases from contaminating the metal depositionsite. When ESD electrode 30 is energized an electrospark is generatedbetween rotary ESD electrode 30 and top surface 16 at edge 20. Theelectrospark generates sufficiently high temperature to cause rotaryelectrode 30 to melt a portion of edge 20 forming a generally roundededge 20, and to deposit a coating 22 (see, e.g., FIG. 6) of electrodematerial alloy at cooling hole 10 adjacent surface 16 of metal substrate12. As indicated by arrow 38, a force may be applied to rotary electrode30 to press the electrode tip 32 into contact with substrate 12 incooling hole 10.

Coating 22 enhances the resistance to oxidation locally around coolinghole 10. In one embodiment, coating 22 may be deposited on the top side,e.g., at the aft end, to enhance the resistance to oxidation. Further,by using the ESD in one the top side 16 only, the rounded hole formedthereby reduces the concentration of stress that would otherwise bepresent at a sharp corner of the cooling hole 10.

In one embodiment, ESD electrode 30 is pressed forcibly on the coolinghole 10 under shielding gas 34. An electrospark 35 is establishedbetween rotary electrode 30 and cooling hole 10 of metal frame substrate12. The electrospark generates local heating and forging of metal framesubstrate 12 and rotary electrode 30. An ESD coating is built up on theexit of cooling hole 10. The exit geometry of cooling hole 10 istailored by the tapered electrode shape. The selection of electrodedepends on the application. Any superior oxidation resistant materialcan be used as an ESD electrode The electrode may be, e.g., a sinteredmetal alloy powder such as CoNiCrAlY, although any oxidation-resistantMCrAlY system or superalloy composition may be used to make the coatingbuild up on the hole exit.

Referring next to FIG. 5, a flow chart is provided to describe themethod of the present disclosure. At step 100, the method begins byproviding a workpiece having a surface including one or more coolingholes. The method proceeds to step 102 providing electro-sparkdeposition (ESD) equipment, and at step 104, providing an ESD torchelectrically connected with the ESD equipment, including an inert gassource and a rotary electrode 30 of a conductive material. At step 106,rotary electrode tapered tip portion 32 is then inserted at leastpartially into cooling hole 10 through metal frame substrate 12. Next,at step 108, the method provides an inert gas curtain around thedeposition site at edge of cooling hole by directing a first shieldinggas flow at tip portion. At step 110 the method optionally provides asecond shielding gas flow at tip portion from bottom surface to preventgases from contaminating the metal deposition site. Next, at step 112,force is applied to rotary electrode 30 to press the electrode tipportion 32 into contact with substrate 12 in cooling hole 10. Then, atstep 114, ESD electrode is energized to generate an electrospark betweenrotary ESD electrode and top surface at cooling hole edge atsufficiently high temperature to melt edge and form a rounded edge anddeposit a coating of electrode material alloy at cooling hole edge.

It should be understood that the application is not limited to thedetails or methodology set forth in the following description orillustrated in the figures. It should also be understood that thephraseology and terminology employed herein is for the purpose ofdescription only and should not be regarded as limiting.

It is important to note that the construction and arrangement of the ESDsystem as shown in the various exemplary embodiments is illustrativeonly. Although only a few embodiments have been described in detail inthis disclosure, those who review this disclosure will readilyappreciate that many modifications are possible (e.g., variations insizes, dimensions, structures, shapes and proportions of the variouselements, values of parameters, mounting arrangements, use of materials,colors, orientations, etc.) without materially departing from the novelteachings and advantages of the subject matter recited in the claims.For example, elements shown as integrally formed may be constructed ofmultiple parts or elements, the position of elements may be reversed orotherwise varied, and the nature or number of discrete elements orpositions may be altered or varied. Accordingly, all such modificationsare intended to be included within the scope of the present application.The order or sequence of any process or method steps may be varied orre-sequenced according to alternative embodiments. In the claims, anymeans-plus-function clause is intended to cover the structures describedherein as performing the recited function and not only structuralequivalents but also equivalent structures. Other substitutions,modifications, changes and omissions may be made in the design,operating conditions and arrangement of the exemplary embodimentswithout departing from the scope of the present application.

It should be noted that although the figures herein may show a specificorder of method steps, it is understood that the order of these stepsmay differ from what is depicted. Also two or more steps may beperformed concurrently or with partial concurrence. Such variation willdepend on the software and hardware systems chosen and on designerchoice. It is understood that all such variations are within the scopeof the application. Likewise, software implementations could beaccomplished with standard programming techniques with rule based logicand other logic to accomplish the various connection steps, processingsteps, comparison steps and decision steps.

While the exemplary embodiments illustrated in the figures and describedherein are presently preferred, it should be understood that theseembodiments are offered by way of example only. Accordingly, the presentapplication is not limited to a particular embodiment, but extends tovarious modifications that nevertheless fall within the scope of theappended claims. The order or sequence of any processes or method stepsmay be varied or re-sequenced according to alternative embodiments.

What is claimed is:
 1. A method for providing a coating comprising:providing a substrate having a first surface and at least one coolinghole; providing a portable coating device including: electro-sparkdeposition (ESD) equipment, and an ESD torch electrically connected withthe ESD equipment, the ESD torch including: an inert gas source; and arotary electrode including a conductive material, the rotary electrodedisposed within the ESD torch, the rotary electrode shielded by an inertgas, wherein rotary electrode applies a compositionally controlledprotective coating to the first surface of the substrate; inserting therotary electrode at least partially into the cooling hole; generating anelectrospark between rotary ESD electrode and the substrate to form arounded edge and deposit a coating of electrode material alloy at acooling hole edge.
 2. The method of claim 1, further comprising pressingthe rotary electrode into contact with the substrate in the at least onecooling hole.
 3. The method of claim 1, wherein the step of insertingthe rotary electrode further comprises inserting a tip portion of therotary electrode into the at least one cooling hole.
 4. The method ofclaim 1, further comprising providing an inert gas curtain around adeposition site at the cooling hole edge by directing a first shieldinggas flow at the rotary electrode.
 5. The method of claim 1, furthercomprising providing a second shielding gas flow at the rotary electrodefrom a bottom surface of the substrate.
 6. The method of claim 1,further comprising applying force to the rotary electrode to makecontact with the substrate in the at least one cooling hole.
 7. Themethod of claim 1, further comprising forming a metallurgical bondbetween the substrate and the alloyed coating on an exit edge of the atleast one cooling hole.
 8. The method of claim 3, further comprisingproviding a transition portion on the tip portion, the transitionportion transitioning from a diameter of the rotary electrode slightlylarger than a diameter of the at least one cooling hole to a tip portionhaving a diameter less than the diameter of the at least one coolinghole to permit partial insertion of tip portion.
 9. The method of claim8, wherein the transition portion comprises a geometry for forming thecooling hole edge.
 10. The method of claim 9, wherein the geometry is arounded edge.
 11. A system for depositing a coating on a cooling holeedge in a substrate, comprising: an electrospark device and an electroderemovably supported in the electrode holder; the electrospark deviceconfigured to apply a coating of a material when inserted into a coolinghole in the substrate and placed into contact with the metal substrate;and a rotary electrode disposed within the ESD torch, the rotaryelectrode shielded by an inert gas, wherein the rotary electrode appliesa compositionally controlled protective coating to the substrate at anedge of the cooling hole in response to an electrospark generated by anelectrical current through the rotary electrode.
 12. The system of claim11, wherein the rotary electrode comprises a partially tapered tipportion, the tip portion configured to be inserted at least partiallyinto the cooling hole.
 13. The system of claim 12, wherein the tipportion comprises a transition portion transitioning from a firstdiameter to a second diameter, the first diameter being equal to adiameter of the rotary electrode larger than a diameter of the coolinghole, and the second diameter of the tip portion being less than thediameter of the cooling hole, wherein the tip portion is at leastpartially insertable into the cooling hole.
 14. The system of claim 12,wherein the tip portion comprises a geometry configured to form apredetermined geometry of an edge of the cooling hole.
 15. The system ofclaim 14, wherein the predetermined geometry comprises a rounded edge.16. The system of claim 11, wherein the substrate comprises a combustionhardware component.
 17. The system of claim 16, wherein the combustionhardware component is a transition piece aft picture frame of a turbineengine, and wherein an edge of the transition piece aft picture framecomprises a plurality of the cooling holes.
 18. The system of claim 11,further comprising a first shielding gas flow directed at the tipportion, the first shielding gas flow configured to provide an inert gascurtain around a deposition site at an edge of the cooling hole.
 19. Thesystem of claim 18, wherein the protective coating is an oxidationresistant layer over substantially the entire surface of the edge of thecooling hole with a thickness up to 30 mils.
 20. The system of claim 18,wherein the electrode alloy comprises, by weight of alloy, from about20.0 to about 82.0 percent nickel, from about 10.0 to about 28.0 percentchromium, from about 5.0 to about 15.0 percent aluminum, up to 1.5percent yttrium, and the balance cobalt and incidental impurities.
 21. Aturbine engine component of a metal frame substrate including coolingholes, wherein at least one of the cooling holes comprises a roundededge with a coating adjacent top surface, wherein the rounded edge withthe oxidation resistant coating is applied using the method of claim 1.